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Aliases are a way of substituting a command for another
command, a command with arguments, or for multiple commands.
Here is a simple example of an alias:
alias dir ls
In this example, whenever dir is given as a command,
the shell will substitute it with the ls command.
Any arguments to the dir alias will be attached to
the substituted command, so dir -l mr.file will turn
into
ls -l mr.file.
Another example:
alias ls ls -l
Here each time you use the ls command, the shell
will substitute it for ls with the long listing
option -l. This alias will also effect other
aliases, so the dir alias will be substituted with
ls, which in turn will be substituted with ls
-l.
Lets take a look at a little more complicated example:
alias mypager "echo first file is \!^; cat -n \!* |
more"
In this example we have quoted the alias, to prevent the
shell from seeing the ';' and trying to pipe stuff to more
when we want that to be done at the time the substitution of
the alias is done. Furthermore, !^ and !*
are escaped so the shell does not do history substitution
now. This alias will create a command called mypager
which will take a list of files as an argument, tell you the
name of the first file, and then display the files using cat.
The "-n" option of cat displays line numbers. The output of
all this is then piped to more, so that it is displayed one
screen at a time.
The shell is smart enough to resolve an alias such as
alias ls ls -l where the substitution to ls
-l, will invoke the command ls, and not
re-substitute ls -l to ls -l -l and so on.
However, csh will let you do the following sequence:
alias dir ls
alias ls dir
which when either dir or ls is executed,
will throw the shell into an infinite loop. Tcsh, on the
other hand, will report an alias loop error, and will not
execute the substitution.
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